At the end of act two of The Crucible, John Proctor's wife has been arrested on the "proof" that she stuck a needle in a doll and that it was some sort of voodoo doll for Abigail Williams. Abby fell down at dinner with the judges and pulled a needle out of her belly--she claimed it was Elizabeth's "familiar spirit" that stabbed her. Proctor can see how absurd this claim is; Mary Warren gave the poppet (or doll) to Elizabeth that day--it was Mary who put the needle there, not Elizabeth. Mary herself admits as much when they question her in the Proctor household. She admits,

"Why--I made it in the court, sir, and--give it to Goody Proctor tonight...ask Abby, Abby sat next to me when I made it."

To any rational human, this is good proof that Elizabeth Proctor did not bewitch Abby; rather, Abby saw Mary stick the needle in the doll and used that as chance to get Elizabeth accused. However, Elizabeth is still arrested and taken to jail. John wants Mary to go to the court and

"tell the court how that poppet come here and who stuck the needle in."

He wants, desperately, to prove his wife's innocence, and believes that if Mary Warren testifies of that, the courts will see reason.